In the year 2448 from Creation (1313 BCE), Tammuz 16 was the 40th day following the Giving of the Torah at Mount Sinai,
and the people of Israel wrongly expected Moses' return from the mountain (he would actually return on the following day).
When their leader failed to return, they demanded from Aaron: "Make us a god that shall go before us".
Hur (Moses' nephew, the son of Miriam and Caleb) tried to stop them and was killed by the mob.
Aaron fashioned a calf of molten gold.